Low attenuation optical waveguide

ABSTRACT

Disclosed is a single mode optical waveguide fiber having a core refractive index profile in which the profile parameters are selected to provide an attenuation minimum. A set of profiles having the same general shape and dimensions is shown to have a group of profiles contained in a sub-set which exhibit a minimum of attenuation as compared to the remaining members of the set. The members of the sub-set have been found to have the lowest effective group index, n geff , and the lowest change in β 2  under waveguide fiber bending.

RELATED APPLICATIONS

[0001] This application is a continuation application filed under 37 CRF1.53(b), which claims priority to and incorporates by reference pendingprior application Ser. No. 09/641,019, filed on Aug. 17, 2000, entitledLOW ATTENUATION OPTICAL FIBER, which application has been allowed, andwas a continuation of U.S. application Ser. No. 09/145,755, filed Sep.2, 1998, which application has now granted and is now U.S. Pat. No.6,134,367, which claimed the benefit of and priority to US provisionalapplication Ser. No. 60/058,774, filed Sep. 12,1997.

BACKGROUND OF THE INVENTION

[0002] The invention relates to an optical waveguide fiber optimized forlow attenuation. In particular, waveguide fiber attenuation is minimizedfor any core refractive index profile by proper selection of the corerefractive index profile variables.

[0003] The dependence of waveguide properties upon the configuration ofthe refractive index profile has been described in the pioneeringpatent, U.S. Pat. No. 4,715,679, Bhagavatula. In that patent, corerefractive index profiles are disclosed which provide for a variety ofwaveguide fiber properties, especially those having a zero dispersionwavelength shifted into the 1550 nm operating window and those whichhave a relatively constant dispersion over an extended wavelength rangesuch as 1250 nm to 1600 nm.

[0004] In response to demands for specialized waveguide fibers,particularly with regard to high performance waveguides, investigationof waveguide core refractive index profiles has intensified. For examplein U.S. Pat. No. 5,483,612, Gallagher et al., (the '612 patent) there isdisclosed a core profile design which provides low polarization modedispersion, low attenuation, a shifted dispersion zero, and lowdispersion slope. Other core refractive index profiles have beendesigned to meet the requirements of applications which include the useof higher power signals or optical amplifiers.

[0005] A problem which may arise when a core profile is altered in orderto arrive at a desired property is that the property is realized at theexpense of another essential property. For example, a certain corerefractive index profile design may provide increased effective area,thus reducing non-linear distortion of the signal. However, in thislarge effective area waveguide fiber, the bend resistance may beseriously compromised. Thus, core profile design is an exacting task, inwhich model studies usually precede the manufacturing stage of productdevelopment.

[0006] The interaction of the profile variables is such that one skilledin the art usually cannot, except perhaps in a very general way, predictthe impact of a refractive index profile change upon such waveguideproperties as, bend resistance, attenuation, zero dispersion wavelength,and total dispersion and total dispersion slope over a selectedwavelength range. Therefore, studies of waveguide refractive indexprofiles usually include a computer simulation of the particular profileor family of profiles. Manufacturing testing is then carried out forthose refractive index profiles which exhibited the desired properties.

[0007] In a continuation of the work disclosed in the '612 patent, afamily of profiles was found which produced a high performance fiberhaving a zero dispersion wavelength above a pre-selected band ofwavelengths and excellent bend resistance. A description of this workhas been filed recently as a provisional application, Ser. No.60/050550.

[0008] As further model studies and manufacturing tests were completed,it became clear that:

[0009] a particular family of profiles could be found to provide aselected set of operating parameters; and, most surprisingly,

[0010] the profiles of the particular family could be further adjustedto optimize attenuation without materially changing the operatingparameters.

Definitions

[0011] The radii of the regions of the core are defined in terms of theindex of refraction. A particular region has a first and a lastrefractive index point. The radius from the waveguide centerline to thelocation of this first refractive index point is the inner radius of thecore region or segment. Likewise, the radius from the waveguidecenterline to the location of the last refractive index point is theouter radius of the core segment. Other definitions of core geometry maybe conveniently used.

[0012] Unless specifically noted otherwise in the text, the parametersof the index profiles discussed here are defined as follows:

[0013] radius of the central core region is measured from the axialcenterline of the waveguide to the intersection with the x axis of theextrapolated central index profile;

[0014] radius of the second annular region is measured from the axialcenterline of the waveguide to the center of the baseline of the secondannulus; and,

[0015] the width of the second annular region is the distance betweenparallel lines drawn from the half refractive index points of the indexprofile to the waveguide radius.

[0016] The dimensions of the first annular region are determined bydifference between the central region and second annular regiondimensions.

[0017] Core refractive index profile is the term which describes therefractive index magnitude defined at every point along a selectedradius or radius segment of an optical waveguide fiber.

[0018] A compound core refractive index profile describes a profile inwhich at least two distinct segments are demarcated.

[0019] The relative index percent (Δ %) is:

[0020] Δ %=[(n₁ ²−n_(c) ²)/2n₁ ²]×100, where n₁ is a core index andn_(c) is the minimum clad index. Unless otherwise stated, n₁ is themaximum refractive index in the core region characterized by a % Δ.

[0021] The term alpha profile refers to a refractive index profile whichfollows the equation,

[0022] n(r)=n₀(1−Δ[r/a]^(α)) where r is radius, A is defined above, a isthe last point in the profile, r is chosen to be zero at the first pointof the profile, and I is a real number. For example, a triangularprofile has α=1, a parabolic profile has α=2. When α is greater thanabout 6, the profile is essentially a step. Other index profiles includea step index, a trapezoidal index and a rounded step index, in which therounding may be due to dopant diffusion in regions of rapid refractiveindex change.

[0023] The profile volume is defined as 2r

1 ^(r2) (Δ % r dr). The inner profile volume extends from the waveguidecenterline, r=0, to the crossover radius. The outer profile volumeextends from the cross over radius to the last point of the core. Theunits of the profile volume are %μm² because refractive index isdimensionless. To avoid confusion, the profile volumes will be connoteda number followed by the word units.

[0024] The crossover radius is found from the dependence of powerdistribution in the signal as signal wavelength changes. Over the innervolume, signal power decreases as wavelength increases. Over the outervolume, signal power increases as wavelength increases.

[0025] The bend resistance of a waveguide fiber is expressed as inducedattenuation under prescribed test conditions. A bend test referencedherein is the pin array bend test which is used to compare relativeresistance of waveguide fiber to bending. To perform this test,attenuation loss is measured for a waveguide fiber with essentially noinduced bending loss. The waveguide fiber is then woven about the pinarray and attenuation again measured. The loss induced by bending is thedifference between the two measured attenuations. The pin array is a setof ten cylindrical pins arranged in a single row and held in a fixedvertical position on a flat surface. The pin spacing is 5 mm, center tocenter. The pin diameter is 0.67 mm. During testing, sufficient tensionis applied to make the waveguide fiber conform to a portion of the pinsurface.

[0026] The bend test used in the model calculations was a single turn ofwaveguide fiber around a 30 mm diameter mandrel.

[0027] The effective group refractive index (n_(geff)) is the ratio ofthe velocity of light to the group velocity. The mathematical expressionfor n_(geff) in terms of electromagnetic field, refractive index,wavelength and propagation constant, derives from Maxwell's equations,or, more particularly, from the scalar wave equation.

[0028] The propagation constant β, also called the effective refractiveindex is an electromagnetic field parameter related to field propagationvelocity and is found by solving the scalar wave equation for a selectedwaveguide. Because β depends upon waveguide geometry, one may expectthat bending the waveguide will change β. An example of a scalar waveequation descriptive of the electromagnetic fields which are supportedby a particular waveguide geometry is found in “Optical and QuantumElectronics”, J. P. Meunier et al., 15, (1983), pp. 77-85.

SUMMARY OF THE INVENTION

[0029] The present invention is therefore directed to an opticalwaveguide fiber having a core refractive index profile which produces apre-selected set of operating properties and in which attenuation isoptimized for that particular refractive index profile.

[0030] The novel core refractive index profile has a core region and asurrounding clad layer which together form a waveguide fiber. To confinelight within the fiber, at least a portion of the core index profilemust have a higher refractive index than at least a portion of the cladlayer. Usually, the clad layer index profile is a single step, althoughuseful designs which have a modified clad index have been made.

[0031] The core refractive index profile, defined above, is a refractiveindex value defined at each point along a specified portion of thewaveguide radius. Thus the core index profile may be expressed as anindex value n(r) at points along a radius beginning at 0, the center ofthe waveguide, and extending to a radius r_(o). This core index isdesigned to produce a pre-selected set of waveguide fiber operatingproperties. The operating properties each may have tolerance limits sothat a family or set of core refractive profiles exists which producethese waveguide operating properties. Even in a model case, in which theoperating properties each have a single value, a set or family ofrefractive index profiles which provide the properties can be found.

[0032] The set of core refractive index profiles, which provide thepre-selected waveguide operating parameters, may be specified by statingthe amount of refractive index variation at any radial point r of therefractive index, δn(r), and the amount of variation of the totalradius, δr_(o), which is allowable.

[0033] Through modeling studies of the family or set of allowablerefractive index profiles, a subset of profiles have been found whichhave lower attenuation than the other members of the set. The waveguideproperties which distinguish this highly preferred subset are theeffective group refractive index, n_(geff), and the propagation constantβ. In particular, the highly preferred subset of lowest attenuationrefractive index profiles have the lowest n_(geff) of any other membersof the set, and exhibit the smallest change in the square of thepropagation constant, β², when the waveguide is bent. Any of a number ofbending models can be used to calculate the bending induced change inβ². A bending model used in the case described here is one in which thewaveguide makes one turn around a 30 mm diameter mandrel.

[0034] The lowest attenuation refractive index profile family or set hasbeen found for step index single mode waveguide fiber, trapezoidalshaped index, rounded step index, and compound index profiles made up ofcombinations of these. Thus it is believed to be very likely thatessentially every family or set of profiles has members which exhibitlowest attenuation and that these members are characterized by thelowest n_(geff) and lowest change in β² in bending as compared to anyother member of the set or family of profiles.

[0035] Thus the core refractive index profile may have relative indexdifferences, Δ, which are positive or negative. The index profiles mayhave only one region of step, trapezoidal, rounded step, or α-profileshape, in which α can assume any real number value. Alternatively, thecore refractive index profile may be any combination or permutation ofthese shapes in two or more regions which are defined segments of thecore region.

[0036] A particular compound core embodiment of the novel corerefractive index profile is one in which N segments are defined. Eachsegment has a Δ % value and a shape. Various widths and radii (see thedefinitions section above) of the segments are defined until thecomplete geometry of the compound core has been specified. For examplethe outer radius, measured from the waveguide center to the outermostpoint of the particular core refractive index segment, of each segmentmay be specified. In general, the relative indexes, Δ %, for single modewaveguide fibers are in the range 0 to 3.5% and the outer radius of theoutermost segment is in the range of 1 μm to 30 μm. A preferred band ofoperating wavelengths is 1200 nm to 1750 nm, which includes theoperating windows near 1300 nm and 1550 nm.

[0037] An embodiment of the invention comprises a compound core havingthree segments. This embodiment is discussed in detail below. The modelused to calculate waveguide fiber structure and properties can beadapted to account for a refractive index dip on centerline. In the casewhere there is some dopant depletion from the centerline, the lowerlimit of Δ₁% is decreased about 15%. Although dopant compensation can bemade to eliminate centerline depletion, it is more time and costefficient to adjust other profile parameters to compensate for thedepletion. The definitions given above are followed in that r₃ is theradius drawn to the center of the base of the third segment and that w₃is the width at the half relative index points of the third segment.

[0038] A preferred embodiment of the three segment core refractiveprofile is given in Table 1. The waveguide parameters in Table 1 providethe waveguide fiber properties set forth on Table 2.

[0039] A second preferred embodiment is given in Table 3. The waveguidefiber having parameters as set forth in Table 3 also give rise towaveguide fiber properties of Table 2.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040]FIG. 1 is a general illustration of the various profile types.

[0041]FIG. 2 is an illustration of the three segment compound coreembodiment.

[0042]FIG. 3 is an illustration of the two segment compound coreembodiment.

[0043]FIG. 4 is a chart of attenuation at 1550 nm versus effective groupindex.

[0044]FIG. 5 is a chart of attenuation at 1550 nm versus the bendinduced change in β².

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0045] Recent study of optical waveguide fiber core refractive indexprofiles has resulted in the description of a large number of coreprofiles which provide unique and advantageous waveguide properties.Examples are U.S. Pat. No. 4,715,679, Bhagavatula and U.S. Pat. No.5,483,612, Gallagher in which the disclosed core index profiles weretailored to provide total dispersion, zero dispersion wavelength, andtotal dispersion slope which fit well with a particular fiberapplication. These investigations combined with additional work withvarious sets of refractive index profiles showed that it was possible todesign waveguide fiber for very high performance systems. For example,waveguide fibers were developed which could accommodatetelecommunication systems capable of high information transmissionrates, using high power lasers, and optical amplifiers.

[0046] Researchers working with novel core index profiles, found that,in general a required set of waveguide functional properties could beprovided by any of one or more sets of profile types. The decision as towhich profile to use in the manufacturing process was driven by ease ofmanufacture, low cost, and insensitivity of the waveguide function tonormal variations in waveguide manufacturing.

[0047] More recent work has shown that an additional factor must beincluded in evaluating which profile is best suited to the intended useand to low cost, high efficiency manufacturing. In particular, thisrecent work has served to identify sub-sets or sub-families of coreindex profiles which are unique in that they provide minimum attenuationcompared to other members of the profile sets or families.

[0048] Experimentation with manufactured waveguides having corerefractive index profiles in both the family and the low attenuationsub-family has shown that the attenuation difference is not due tomanufacturing variability, Rayleigh scattering, or

[0049] —OH content. The difference in attenuation between the profileset and the related profile sub-set stems from the details of theprofile shape and so is termed “profile attenuation”.

[0050] The novel feature of the core refractive profiles disclosed anddescribed herein is that they are members of their respective minimumprofile attenuation sub-sets.

[0051] Parametric modeling studies and experimentation with manufacturedwaveguide fibers showed that profile attenuation is correlated witheffective group refractive index, n_(geff) and propagation constants β.More particularly, the studies showed that minimum profile attenuationwaveguide fibers have minimum n_(geff) and minimum change in β² inbending of the waveguide. This unexpected result provides anotherimportant tool in designing optimum core refractive index profiles foressentially all types of telecommunications uses.

[0052] The general type core refractive index profile of this inventionis illustrated in FIG. 1. Note that the reference for the relative indexis the clad layer index. The solid line index profile has a centralpoint 24 of relatively low relative index percent, Δ %. The profileportion of higher Δ %, 6, may be, for example, an α-profile or a roundedstep profile. A flat portion of the profile, 14, is followed by anotherlower Δ % portion, 18, whose relative index is negative. Anotherα-profile or rounded step profile region, for example, 20, followsregion 18. The dots, 22, indicate the profile may include additionalannular regions. Dashed lines 8 and 10 indicate alternative profileshapes close to the core center. Dashed line 2, a step index profile, isan alternative to the α-profile shape 6. Dashed lines 12 and 16 showalternative profile shapes for the negative Δ % region of the profile.

[0053]FIG. 1 also shows the definitions of radii and width as the termsare used herein. Radius 3 of the center profile is the line whichextends from the core centerline to the point at which the extrapolatedprofile 6 meets the x-axis. The radii of the annular regions surroundingthe center profile are in general measured from the core center to thecenter of an annular region as illustrated by radius 7 of annulus 20.The width of an annular region is taken at the half Δ % points asillustrated by width 5 of annulus 20.

[0054] Extensive modeling studies as well as manufacturing studies weredone on the profile illustrated in FIG. 2. Table 1 lists the parametersof the low attenuation sub-set of the profile in FIG. 2. TABLE 1Parameter Upper Limit Lower Limit Δ₁% 1.30 0.77 r₁ (μm) 3.41 2.04 Δ₂%0.16 0 Δ₃% 0.51 0 r₃ (μm) 10.21 5.53 w₃ (μm) 5.76 0 Inner Profile Volume3.62 2.67 (% μm²) Outer Profile Volume 7.86 1.00 (% μm²)

[0055] The cases where either r₃ or W₃ are zero are simply additionalexamples of optimum attenuation for the core refractive indexillustrated in FIG. 3. The definitions of the parameters given in Table1 are found in FIG. 2. The center α-profile, having an α of 1 is shownas curve 30. The refractive index on the centerline, 28, is less thanthe maximum index of α-profile 30. Dashed line 26 indicates that theprofile can be modeled in cases where the maximum index lies on thewaveguide centerline. The relative index of 30 is Δ₁% and the radius 31is r₁. The relative index of region 32 is Δ₂%. The relative index of therounded step 34 is Δ₃%, the radius 33 is r₃, and the width 35 is w₃. Thecore refractive index profiles having the parameters shown in Table 1,may produce the waveguide fiber functional properties given in Table 2.Over 700 core refractive index profiles taken from Table 1 were found tohave the required functional properties stated in Table 2. It will beunderstood that not all combinations of Table 1 parameters will producethe functional properties stated in Table 2. TABLE 2 Parameter UpperLimit Lower Limit Dispersion Zero (nm) 1595 1575 Dispersion Slope(ps/nm² - 0.10 — km) Mode Field Diameter (nm) 9.1 7.9 Cut off Wavelength(nm) 1500 — Pin Array Bend Loss (dB) 8 — Att 1550 (dB/km) 0.203 —

[0056] The waveguide properties shown in Table 2 are characteristic of awaveguide fiber for use in a multiplexed, high input powertelecommunications system. This choice of example was made forconvenience and in no way limits or defines the invention.

[0057] Another core refractive index profile shape was modeled to findparameter limits which would provide the waveguide properties given inTable 2. This second core refractive index shape is shown in FIG. 3.Again we choose to use the center profile shape in which centerlineindex 38 is less than the maximum index of α-profile 40, where α=1.Dashed line 36 indicates that the profile may be modeled without thelower refractive index on centerline. The core refractive index profileshown in FIG. 2 has two segments. The center segment 40 has relativeindex Δ₁% and radius 41, designated r₁ in Table 3. The step portion ofthe index profile, 42, has radius 43, designated r₂ in Table 3. Therelative index of segment 42 is Δ₂%. Note that the outer end point of r₂is found by extrapolating the descending portion of segment 42 to thehorizontal or x-axis. TABLE 3 Parameter Upper Limit Lower Limit Δ₁% 1.251.02 r₁ 2.38 1.84 Δ₂% 0.10 0.03 r₂ 10.54 6.50 Inner Profile Volume 3.352.76 % μm² Outer Profile Volume 7.77 2.24 % μm²

[0058] All possible combinations of the parameters in Table 3 do notprovide waveguides having the properties given in Table 2. However, inthe model study over 200 refractive index profiles which werecombinations of the Table 3 parameters did provide a waveguide havingproperties in the ranges shown in Table 2. The FIG. 2 index profiles ingeneral produced lower dispersion slope, an average of about 0.01ps/nm²-km lower, than the profiles exemplified by FIG. 3.

[0059] Experimental results from attenuation measurements on waveguideshaving four distinct profile types are shown in FIGS. 4 and 5. Waveguidefiber types A, C, and D are variations on the profile shown in FIG. 2.All are dispersion shifted single mode waveguide fiber. The type Awaveguide is further characterized in Table 1. Waveguide fiber B is astep index single mode waveguide fiber.

[0060] In FIG. 4, the attenuation at 1550 nm is charted versus theeffective group index, n_(geff), of each of the waveguides. The processwas carefully controlled to remove any data scatter due to manufacturingvariables. Data scatter due to —OH content effects and Rayleighscattering were also removed. Thus, the clusters of points for eachwaveguide type show the change in attenuation due to a change in theindex profile which is manifested as a change in the effective groupindex. The step index waveguides B, dark squares 44, show a profileattenuation variation of about 0.013 dB/km for the change in n_(geff)shown. Likewise the A waveguides, dark diamonds 48, show a 0.02 dB/kmchange, the C waveguides, dark triangles 46, show a 0.015 dB/km change,and the D waveguides, light triangles 47, show about a 0.017 dB/kmchange in attenuation.

[0061]FIG. 5 shows the same data except that the change in attenuationis charted versus change in β² induced by making a single turn of thewaveguide about a 30 mm mandrel. Here the B step index waveguides, darksquares 54, show about the same change as before. The A type waveguides,having a profile similar to that of FIG. 2, dark diamonds 52, show amuch higher change in attenuation with bending change in β² than do theother FIG. 2 type profiles, i.e., C type, dark triangles 56, and D type,light triangles 50.

[0062] The major finding of the experimental data set forth in FIGS. 4and 5 is that:

[0063] profile attenuation occurs for widely different profile shapes;and,

[0064] profile attenuation is closely related to n_(geff) and change inβ² with bending. Based in these results, one is led to the conclusionthat profile attenuation is essentially a universal phenomenon.

EXAMPLES Manufactured Waveguides of the Type Illustrated in FIG. 2

[0065] Two distinct draw preforms were made in accordance with the corerefractive index profile shown in FIG. 2. The parameters of the twoprofiles are set forth in Table 4. TABLE 4 Parameter Draw Preform #1Draw Preform #2 Δ₁% 0.868 0.864 r₁(nm) 2.773 2.781 Δ₂% 0.023 0.025 Δ₃%0.258 0.216 r₃(nm) 6.71 7.51 w₃(nm) 0.67 0.64 Inner Volume (% μm²) 3.023.08 Outer Volume (% μm²) 3.90 4.01

[0066] The optical properties of the waveguides produced from thesedraws preforms were well within the specified limits as shown in Table3. Some of the waveguide measurements are shown in Table 5. Note thevery low attenuation in both the 1310 nm and the 1550 nm operatingwindows. These waveguides are thus in the subset of low profileattenuation waveguides. TABLE 5 Parameter Draw Preform #1 Draw Preform#2 Dispersion Zero (nm) 1582.5 1584.5 Dispersion Slope 0.077 0.073(ps/nm²-km) Mode Field Diameter 8.34 8.22 (nm) Cut Off Wavelength 11861190 (nm) Att 1310 nm (dB/km) 0.371 0.372 Att 1550 nm (dB/km) 0.1990.201

[0067] These results clearly demonstrate the accuracy and integrity ofthe model and the excellent reproducibility of the process. Theexistence of a low profile attenuation sub-set had been established anda means to manufacture waveguides which lie in the sub-set have been setforth.

[0068] Although particular embodiments of the invention have herein beendisclosed and described, the invention is nonetheless limited only bythe following claims.

We claim:
 1. A single mode optical waveguide fiber comprising adispersion zero wavelength which is >1575 nm; a dispersion slope whichis <.1 ps/nm²-km; a mode field diameter >7.9 and having a refractiveindex profile having at least three segments having refractive index Δ₁,Δ₂, and Δ₃, wherein Δ₁ is >Δ₃ is >Δ₂ and the refractive index profile ofthe core is selcted to result in an attenuation which is <.203 db/km, at1550 nm.
 2. The single mode optical waveguide fiber of claim 1 whereinattenuation at 1550 nm is less than or equal to 0.19 dB/km.